Since its inception in the mid-1970s, real-time echocardiography employing phased array principles has had a significant impact on the practice of medicine particularly in cardiology. The real-time or live nature of image formation is one of the principal advantages of echocardiography next to its portability to the patient bedside and relatively low cost as compared with MRI and CT. Currently, live 2-D image scan rates of typical echocardiograms of 80° to 90° fields of view are 30 to 60 per second. These scan rates are adequate for many cardiac anatomical and functional diagnoses but are inadequate for studies of electromechanical coupling events in the heart. Electrical activity as measured by EKG should be sampled at rates of 500 Hz or greater for diagnostic purposes. To study the interaction of electrical and contractile events with comparable temporal resolution, imaging at 500 Hz (i.e., 500 frames per second) would be required.
There have been several studies in the recent past to increase ultrasound acquisition speeds As cardiac ultrasound is a pulse-echo imaging technique, the maximum frame rate (FR) achievable is ultimately limited by the speed of sound in tissue. For 1-D imaging techniques, such as A-mode or M-mode, the temporal sampling is determined by the maximum range being imaged. When extended to 2-D imaging, the field of view, or number of image lines acquired, must also be factored into the FR. For traditional pulse-echo B-mode imaging, the maximum achievable FR, and thus the maximum temporal sampling rate, is the inverse of the product of the time of flight for one transmit-receive operation and the total number of transmit-receive operations to generate one image. In conventional echocardiography, FR can only be increased by decreasing the resolution, the range, or field of view of the image. Scanning a volume further reduces the achievable imaging rates, in terms of volumes per second, since more look directions are needed to insonify and fully sample the volume. The volume can be thought of as consisting of a number of conventional B mode planes stacked one above the other. This stack defines the volume scanned and is the 3D field of view. Since multiple planes must be scanned 3D ultrasound scanning in adult echocardiography has been limited to about 20 volumes per second or less.
Three independent methods have been used to increase temporal sampling without significantly reducing image resolution and without a reduction of image size. The first method used to increase FR is parallel receive processing, known as exploso scanning. In exploso scanning, the transmit beam is broadened to insonify a larger area and multiple image lines are received from a single transmitted beam. This approach was first applied to echocardiography in 1984. Methods have been explored to broaden the transmit beam, including using a reduced transmit aperture, transmitting an unfocused beam, and defocused transmit beams. The acoustic pressure in the broadened beam may lower the echo levels and reduces the resolution in transmit. However, the overall resolution of the image may not be greatly affected if resolution in receive is maintained.
A second method uses multiple transmit beams, either at the same time or in quick succession. While this method provides an additional increase in FR, cross-talk between the simultaneous beams may lead to increased noise and potential artifacts in the resulting image. Recent work has described methods for reducing crosstalk between beams by various methods, including spatial separation, spectral separation or frequency multiplexing, and various apodization schemes. Crosstalk between beams may exist depending upon the shape (apodization) and separation of the beam. Such crosstalk may lead to image artifacts, such as bright targets appearing in multiple locations in the image. For a typical sector scanned image used in echocardiography acquired using a rectangular aperture array that maintains adequate sampling (i.e., sampled at least twice per diffraction limited resolution cell), the FR is given by:
                              F          ⁢                                          ⁢          R                =                                            N                              T                ⁢                                                                  ⁢                x                                      ·                          N                              R                ⁢                                                                  ⁢                x                                                                                        (                                                      2                    ⁢                                                                  R                        max                                                                    c                        tissue                                                                              +                                      t                    1                                    +                                      t                    3                                                  )                            ⁢                              (                                                      2                    ⁢                                                                                  ⁢                    F                    ⁢                                                                                  ⁢                    O                    ⁢                                                                                  ⁢                    V                                                                              sin                                              -                        1                                                              ⁢                                          λ                      D                                                                      )                                      +                          t              2                                                          (                  Equation          ⁢                                          ⁢          1                )            where λ is the wavelength of sound in tissue, D is width of the rectangular aperture in the scanning plane, Rmax is the maximum range, and FOV is the desired sector field of view in degrees with the arcsine operation also in degrees. NTx is the number of parallel transmit beams, and NRx is the number of received image lines for each transmitted beam. Additional delays t1 and t2 represent the machine dependent turn-around time between sequential transmit-receive operations and between sequential frames, respectively. The third delay, t3, is an additional delay added if multiple sequential transmit beams are temporally serialized. For conventional scanning, both NTx and NRx are 1, and t3 is 0 seconds. For 3D scanning multiple planes separated from each other in the third dimension must be scanned so that the volume scan rate, VR, becomes VR=FR/P volumes per sec, where P is the number of planes scanned to provide the 3D field of view.
The third technique is gated image acquisition. In a gated system, the overall image sector is divided into M smaller sectors. Each small sector is imaged for one cardiac cycle at a higher rate using the patient's EKG signal as a timing reference. The overall image is then created by imaging over M cardiac cycles. This method provides an increase in temporal sampling by a factor of M, but not an increase in overall FR. This technique is sensitive to operator motion and patient motion, which lead to artifacts among the partial sectors. Due to the unpredictable nature of many arrhythmias, gated acquisition is not suited for hearts with arrhythmic events or irregular rhythm.
Using one or a combination of these methods, other investigators have recently studied high-acquisition-rate ultrasound imaging with post-acquisition image formation and display. These developments have not resulted in an instrument capable of live image formation; rather, image formation is off line, which is unsuitable for cardiac clinical imaging.